1. Field of the Invention
The invention relates to a static switch adapted for controlling the opening and closing of an AC electric circuit including an inductive load. The static switch considered includes a component of the inhibitable thyristor type having a gate control circuit and a protective network against voltage gradients.
By inhibitable thyristor will be meant hereafter any controllable turn-off PNPN semiconductor structure either of the bipolar type called GTO (gate turn-off), or of the MOS reaction bipolar type for example known under the commercial names IGT, COMFET, or BIFET, or of the static induction type (SIT). From this application inhibitable thyristors are obviously excluded thyristors properly speaking (SCR).
By gate will be meant not only the gate of a GTO or SIT component but also the MOS gate of an MOS bipolar reaction component.
2. Description of the Prior Art
As is known, precautions must be taken for protecting such components against forward voltage increase gradients following turn-off since this latter only takes place exceptionally at current zero.
For this, it is customary to associate in parallel with the anode-cathode path of the inhibitable thyristor a protective network formed of the series connection of a capacity C and means for charging and discharging this capacity, these means being formed by a parallel resistor R--diode D circuit (RCD network or "snubber"). The capacity charges via the diode during turn-off by attenuating the increased slope of the anode-cathode voltage of the inhibitable thyristor, then is discharged into the resistor at the time of turn-on of the inhibitable thyristor.
It has already been proposed to form choppers using inhibitable thyristors having an RCD network for protection against voltage gradients.
It is however difficult to transpose such static apparatus to controlling the opening and closing of an AC circuit with inductive load while providing correct protection for the inhibitable thyristor against the voltage surges generated at the time of turn-off thereof because of the discharge current of the inductance and/or of overload or short circuit currents.
To construct such a static switch, it is known to associate two inhibitable thyristors in an antiparallel circuit, each thyristor being provided with its own RCD protection network. It has however proved that, when the antiparallel circuit is connected to an AC circuit, its reverse voltage resistance must be ensured by specific components when inhibitable thyristors are used having a low reverse voltage resistance as is the case of most of the inhibitable PNPN components available commercially.
In addition, a leak current flows through the capacity of one of the RCD protection networks. Now this is inadmissable for a static switch controlling a low voltage load.
In the published patent application JP No. 59-184 412, a reverse voltage resistance diode is provided in series or in parallel with each inhibitable thyristor, but this does not solve the problem of the leak current when the switch is disabled or inhibited.
To avoid this leak current, it has been proposed to provide two inhibitable thyristors in an antiparallel circuit at the terminals of the AC circuit, this antiparallel circuit being itself disposed in parallel with a full wave rectifier bridge whose branches include diodes alternately representative of the D element of the RCD network.
At the DC terminals of the bridge are disposed a capacity forming the C element of the RCD network and a series circuit including a resistor forming the R element of the RCD network and an auxiliary thyristor which may be turned on for discharging the capacity.
The advantage of this switch is to avoid a permanent leak current. It has however the drawback of requiring two inhibitable thyristors with identical forward voltage behavior, which goes hand in hand with a high price and with difficulties in obtaining such components. In addition, it is necessary to provide, in addition to two power supplies and two gate control circuits, an isolated control for the auxiliary thyristor.